Rectifier



Patented Sept. i, 1931 UNITED STATES PATENT OFFICE HARVEY C.RE1lTSCHLER, OF EAST ORANGE, AND WILLIAM W. MERRYMON, OIE GLEN RIDG, NEWJERSEY, ASSIGNORS '10 WESINGHOUS. LAIMP COMEANY, A CORPORA RECTIFIERApplication filet! July 20,

This invention relates to an electron discharge device and moreparticularly to a glow discharge device employing a rarefied gasconsatmosphere.

The invention is especially concerned w1th the production of a gasconsconduction rectifier of alternating current, employing a 1nonatomic gasfilling, such as helium, argon or neon or a mixture of such gases, anddepending at least in part, upon the point to plate discharge principlefor rectification.

Hitherto, considerable difiiculty has been experienced in producingalternating current rectifiers of this type capable of operation atrelatively high potentials, which depend on gas ionization forconduction of the current and are independent of thermionic omission.Among the difiiculties which may be mentioned, is the maintenance of themonatomic gas in a pure and uncontaminated condition. Slight traces ofgaseous impurities, such as hydrogen, nitrogen, etc., liberated from theelectrodes, the glass parts of the device or the insulators greatlyincrease the break down voltage and the voltage drop through the deviceand if permitted to accumulate eventually cause the device to becomeinopeTative at the normal voltage impressed across the terminalsthereof.

It has been proposed heretolore to maintain the gas in a pure form byemploying certain clean-up agents which react with the commori gases toform solid compounds Of low vapor pressure. Certain of these mate rials,however, contain considerable quantities of undesirable gas whichisgiven ofi' when the material is vaporized and is not subsequentlyeliminated by the getter material. For instance, magnesium, which ismost commonly employed as a gas clean-up, usually contains anappreciable quantity of hydrogen which is liberated when the getter ishated. Magnesium does not' clean up hydrogen efliciently, however, andconse-' quently, both the residual and liberated hydrogen are likely toremain in the tube as a gascons impurity.

However, even "though the monatomic gascons filling is retrderedextremely pure during the manufacture of the device, never- 1927. SerialNo. 207,073.

theless, with the electrode materials ordinarily employed, sufficientgases are given off by the electrodes and parts of the tube such asinsulators, etc., during operation, probably due to bombardment by thedischarge, to contaminate the monatomic as and greatly increase theresistance an f voltage drop through the device. V

It is one of the objects of the present invention t0 overcome the abovementioned difiiculties and to produce a gascons conduction device-inwhich the gaseous filling remains substantially nated to an objectionale extent over a much longer Period of operation and under more severeconditions of operation than has herefore been attainable.

Another object is to produce a device of the above mentioned characterin whichthe electrodes Will be composed of a material or materials whichfacilitate the maintenance of the gas in a pure condition.

Another object is to proVide a clean-up agent which is effective toremove substantially all traces of all of the common gases and which incombination with the materials of which the electrodes are composed,Will maintain the gascons filling extremely pure.

A still further object is to provide and continuously maintain on thesurface of one of the electrodes a thin deposit of a material which Willdecrease the potential drop at the surface of the electrode.

Still another object is to Provide a metliod of continuouslv maintainingsuch thin dep0sit on the electrode throughout the life of the device.

Another object is to produce a gascons conduction device which iscapable of assing a discharge of relatively high voltage and highamperage with a low and constant potential drop therethrough and inwhich the operating characteristics thereOf will remain constant over along period of opefia tion.

Other objects and advantages Will hercinafter appear.

We have discovered that when certain materials are employed for thecathode and anode of a gascons conduction device toure and uncontami-' vgether with an appropriate clean-np agent, the gascons filling may bema1nta1ned in a substantially pure condition over a much longer periodof time than has been possible heretofore under the same severeconditions of operation.

More specifically, we have found that when the cathode or plateelectrode of a point to plate discharge device, such as a rectifier, iscomposed of one of the refractory metals such as molybdenum or tantalumand the active portion of the anode 1s composed of an active metal suchas thorium, uranium or titanium and a clean-up or gas purifying agent,such as misch metal is employed, the gascons filhng does not becomecontaminated during operation of the dev1ce and the operatingcharacteristics thereof remain substantially constant over many bundredsof hours of operation. Moreover, the voltage drop through the device isconsiderably lower after a few hours operation than When employing suchmetals as nickel, iron, etc., for the electrodes.

In order to further decrease the voltage drop through the tube, we mayemploy a deposit of an electro-positive metal, such as an alkaline earthor alkali metal on the cathode. In order to render such materialeffective throughout the life of the device, it is necessary tocontinuously replace the deposit during operation of the device. Simplycoating the cathode itself with one of these materials is notsufiicient, since the electro-positive metal is soon used up orvaporized away and is no longer effective.

Tubes, the plates of which have been coated r with such metals, whilethey show good characteristics on starting, do not maintain the same fora sufliciently long period of time to be of practical value.

Th manner which we have found most eflective for maintaining a depositof an electro-positive met-al on the cathode is by providing a quantityof a compound of an alkaline earth or alkali metal on the anode, whichcompound during operation is slowly reduced by the material of which theanode is composed to liberate the metal. This metal is then vaporizedover on to the cathode and forms the desired deposit thereon. Thethorium or uranium anodes may be dipped or otherwise coated with asolution of an alkaline earth compound such as barium or calcium nitrateor an alkali metal compound such as sodium or potassium nitrate. Duringthe seasoning of the device, these nitrates are probably converted to anoxide and during the operation of the tube, these oxides are slowlyreduced to metal. This metal is continuously vaporized on to thecathode, thus giving and maintaining a low starting and operatingpotential. VVe prefer to employ the alkaline earth metals, although thealkali metals are also satisfactory. It is necessary, however, that themetal itself and not the oxide thereof, comprise the active deposit onthe cathode. For 1nstance, in certain experiments when barium oxide wasapplied directly to one side of the cathode it prevented the dischargefrom taking place with such coated surface and caused all the dischargeto take place with the uncoated surface until after the oxide had brokendown to metal. Vhen employing molybdenum plates, any oxide which iscarried over from the anode to the cathode is broken down by thereducing action of the metalwhich thereby assists in maintaining anactive film of the clectropositive metal thereon. This is not truc,

however, with such metals as iron or nickel when employed as the platematerial.

It is essential that a compound be employed on the anode Which has arelatively low vapor pressure and which is not too active chemically, sothatit will not be vaporized from the anode toc rapidly. Such compoundsas barium azide, for instance, do not serve efi'ectively, because theycomplotely vaporize away from the anodes very rapidly and are then nolonger available to maintain the cathode deposit.

In place of dipping or otherwise coating the anodes, the alkaline earthor alkali compounds may be incorporated directly in the anode materialduring manufacture the1eof, but in doing, care must be taken to preventthe compound from being reduced and vaporized away during suchmanufacturc.

The gas employed depends upon the output voltage desired. If a highvoltage, that is, a voltage of from 200 to 300 volts is de sired, themost suitable gas appears to be helium. With helium the inverse currentis less than for either argon or neon under the same conditions andhence the tendency t0 arc is less. If lower voltages are desired, argonor neon are quite satisfactory and more eflicient.

In order that the invention may be more fully understood, reference Willbe had to the accompanying drawings, in which:

Fig.- 1 illustrates a gaseous conduction rectifier partly in sectionembodying the features of the present invention; and,

Figs. 2 and 3 illustrate circuit arrangements employed in seasoning thedevice.

The device comprises an envelope 1 containing a pair of anodes 2 and 3,a cathode 4 and a filling of a monatomic gas, such as helium, argon orneon or a mixture of such gases.

The anodes 2 and 3 are preferably'composed of thorium, uranium ortitauium and are carried by support wires 5 of nickel or other suitablemetal secured to leading-in conductors 6 and 7 sealed through the press8 of the device. The anodes may be sep recess 11 in the'lower end of thelavite sleeve and serves to support the same.

Thecathode4is of cylindrical shape and preferably is formed of arefractory metal, such as molybdenum, althoufgh any other convenientshape may be employed, giving the requisite surface area. The anodes 2and 3 are illustrated as projecting into the interior of the cylindricalcathode, but it is to be understood that, if desired, the anodes may bedisposed externally of the cathode. By 'disposing the cathode about theanodes, it may be brought nearer to the press and the support Wires madeshort and direct,thereby producing a more rigid structure. The cathodeis supported from the press by support wires 12 and 13 welded thereto,the former of which is joined to a leading-in' conductor 14 forsupplying current thereto.

The leadingin conductors 6, 7 and 14 are connected 'to the terminalprongs 15, 16 and 17 res}3ectively of the base'l8.

A piece of misch metal or other metal of the -cerium group of rare earthmetals 19 -i s secured to the cathode by a strap 20 welded to theexterior of the cathode. The cathode is heated during exhaust by highfrequency induction currents, to drive out the gases therefrom andduring such heat treatment, the misch metal is vaporized and deposits ina thin film on the bulb. This thin deposit of misch metal assists inmaintaining the gascons filling pure throughout the life of the device,

The envelope is filled with monatomic gas, preferably helium, at-apressure from 5 to 20 mm. We prefer to employ about 12 mm. for an outputvoltage of from 200 to 300 volts and from to 100 milliamperes.

However, if lowef output voltages are desired, argon or neon at a lowerpressure may be employed.

The device after corhpletion should be seasoned to develop the desiredstarting and operating' characteristics.

The seasoning treatment which we have successfully employed is conductedin two parts and is best understood by reference to Figures 2 and 3. Thetube 20 is first operated foi about 10 minutes directly-across a 220volt A. C. line 21 with the anodes 2 and 3- in series with 15 watt, 110volt tungstean filament lamps 22 and 23.

The sesoning is then continued with the anodes 2 and 3 joined togetherand con-4 nccted to one side of a source 24 cf direct current at from400 to 500 volts, with the cathode 4 joined to the other side of theline through a filter circuit 25 containing a load 26. The tube shouldbe ope'rated in this circuit until the anodes become heated to a brightrcd heat and thrc is no appreciable fiashing between the anodes and thecathode, at a voltage in excess ofthat which they will receive in use.

With a rectifier of the above construction, that is, employing thoriumanodes, a molybdenum cathode, mish metal getter and a filling of heliumat about 12 mm. of mercury,an output voltage of around 200 volts at 85milliamperes may be continuously maintained over many hundreds of hours.VVheh employing plates of such metals as .iron or nickel and exactly thesame transformer and filter circuit, an output of only about 150 to. 160volts or 85 milliamperes can be obtained after operating for about 100hours or s0.

In order to Still further reduce the potential drop through the device,an alkaline earth or an alkali metal may be employed 'as a thin depositon the cathode. In order to maintain this deposit throughout the life ofthe device, it is essential to provide a reserve supply of such metalwithin the clevice and to continuously supply it to the surface ofthecathode during the operation of the rectifier. Merely coating thecathode with s'uch metal is not satisfactory, since it rapidly vaporizesaway and is no longer available. Therefore, in order to provide a"reserve supply of such metal, we-

coat the anodes 2 and 3 with a compound of an alkaline earth or alkalimetal such as' barium nitrate which is probably convertcd to oxideduring seasoning, which slowly reacts with the thorium duringtheoperation of the device to liberate the barium metal.

The barium is vaporized by the discharge and is continually supplied tothe cathode. The reaction of the compound with the thorium is slow sothat the compound is not entirely used up during the normal life of thedevice.

The device of the present invention has bcen designed particularly as arectifier for supplying plate current for radio receiving apparatusthrough the so-called B-eliminator apparatfls. In such a use, the outputcurrent is smoothed out by any appropriate filter circuit embodyinginductance and capacities. In such circuits, the charge which builds upon the anode when the same is negativeis several times that on thecathode When the cathode is negative, due to the action of thecondensers in the filter circuit. Thus, for instance, with an outputvoltage of around 200 volts, the charge on the anode during the inversecycle may amount to as high as 700 volts. Due to this high voltage onthe anodes, the small inverse ouvrent which fiows between the cathodeand anode bombards the anode at a high velocity and materially assistsin the reduction of the alkaline earth or alkali compound on the anode.

Obviously, other means may be employed for maintainingthe active depositon the cathode, but we have found the method described above to behighly successful.

VVhile We have shown and described but a single embodiment of ourinvention, it is to be understood that many changes and modificationsmay be made therein without departing from the invention.

\Vhat is claimed is:

1. An electrode for a gaseous conduction device composed of thorium anda nitrate of a metal of the class including the alkaline earth and thealkali metals.

2. An electrode for a gaseouS conduction device composed of thoriumhaving a coating of barium nitrate thereon.

3. The method of producing and maintaining a deposit of anelectro-positive metal on the cathode of a gaseous conduction device,comprising fomning the anode of thorium, coating said anode with acompound of the electro-positive metal decomposable by said anode andpassing a discharge through the device to decompose the compound,liberate the electro-positive metal and vaporize the same from the anodeonto the cathode.

4. A gaseous conduction rectifier comprising an envelope containing amonatomic gas, a cathode operable below incandescence, composed ofmolybdenum, an anode composed of thorium, a gas purifying agentincluding a metal of the cerium group of rare earth metals and a coatingof a compound of an electropositive metal on the anode.

5. A gaseous conduction rectifier comprising an envelope containinghelium gas at a pressure from 5 to 20 mm., a cathode composed ofmolybdenum, an anode composed of thorium, a gas purifying agent composedof a metal of the cerium group of rare earth metals and a coating ofbarium nitrate on the anode.

In testimof1y whereof We have hereunto subscribed our names this 19thday of July HARVEY C. RENTSCHLER. WILLIAM W. MERRYMON.

